The FW method, which has advantages capable of enhancing fiber strength and elastic modulus to the maximum and being reliable in an accurate laminating direction, is a method of forming a predetermined shape by impregnating a continuous reinforcing resin (“continuous fiber” hereinafter) with resin as winding the continuous fiber at an arbitrary winding angle around a cylindrical mandrel rotating. Based on characteristics required to a tubular body produced by the FW method, the winding angle (fiber orientation) of fiber with respect to the mandrel should be arbitrarily designed. Therefore, it is desirable that the winding angle is designed arbitrarily between 0° of being parallel with the mandrel axis and 90° of being orthogonal thereto.
In forming a tubular body by the FW method with a cylindrical mandrel, it has been usual that the winding angle is around 30° at the lowest to prevent the fiber yarn from slipping. However, the winding angle is sometimes required to be lower than 30° to give certain characteristics to a product.
To achieve such a low angle, the FW method can be performed as a conventional forming method in which a continuous fiber impregnated with resin is wound around the mandrel shown in FIG. 4(a) of which mandrel body 20 is provided with antislip jigs 21 at which the continuous fiber turns back on both ends as being wound at an appropriate angle. JP-H6-26858-B discloses a technique of a tubular body forming method by the filament winding method, in which forming products are cut at a position of ring-shaped antislip jigs 21, which are fitted on mandrel shaft section 22 and have dry-wooden pins to contact an end of the mandrel body.
In that technique, antislip jig 21 has pins to hook the continuous fiber to be wound so that the continuous fiber is stably wound without slipping regardless of the winding angle. The continuous fiber hooked by the pins turns back and therefore an excessive tension might be caused widthwise to decrease the fiber width of the continuous fiber. To produce a tubular body having a desirable quality by being thermally cured after finishing a winding process, formed products must be cut at considerably inner position from the pins, so that a yield might be worsened.
To solve the problem, a conventional FW method without antislip jigs is known, which enables a low-angle winding together with good yield and good lamination. JP-H3-281228-A discloses a tubular body forming method performed with a mandrel used for the filament winding in which mandrel body 20 on which continuous fiber 23 is directly wound is integrally provided with shaft section 22 having a smaller diameter and step section (shoulder section) 24 on which the continuous fiber is wound between the body section and the shaft section, as shown in FIG. 4(b). It is preferable that the mandrel satisfies Formula (1), where D implies body diameter, d implies shaft diameter and a implies winding angle.sin α=d/D  (1)
In this technique, fiber slippage may be a problem specifically if the winding angle α is close to 0° or a surface of the mandrel step section is curved to prevent the continuous fiber from breaking. In addition, a fiber slippage toward the middle of the mandrel may be a problem if the winding angle a increases during winding the fiber on the step section.
To solve the above-described problems of conventional arts, antislip jigs to prevent the continuous fiber from slipping should be provided at both ends of the mandrel to perform a winding stably at any winding angle.
Accordingly, it could be helpful to provide an FW method producing a tubular body, which prevents a yarn width from decreasing at the time of turning back of the continuous fiber, improves a product yield, and enables a low-angle winding under a good lamination, even if the FW method is performed with antislip jigs.